This invention relates to pesticidal mixtures of carbamate compounds and synthetic pyrethroids, and more particularly to improving such mixtures by the addition of certain stabilizers to prevent the racemization of the pyrethroid to a less pesticidally-active form.
Carbamate compounds and synthetic pyrethroids are well known for their control of agricultural pests such as insects and acarids. Although these pesticides are active by themselves, their activities are often selective, being higher against some agricultural pests than against others. Accordingly, various carbamate compounds, such as methomyl, and synthetic pyrethroids have been formulated together, providing mixtures that are more advantageously active against a broader spectrum of pests. Such mixtures are shown, for example, in German Patent DT No. 2805050, published Aug. 10, 1978.
Synthetic pyrethroids usually have one or more asymmetric carbon atoms and consequently exist in two or more stereoisomeric forms. It is well known that not all stereoisomers of a particular pyrethroid possess the same degree of pesticidal activity; usually, a specific isomer or diastereomer pair are more active than is a racemic mixture of all the isomers.
It has been found, however, that some of the synthetic pyrethroids, when resolved or partially resolved to their more pesticidally active isomers, can undergo racemization under certain conditions, reverting back to a less active form. The racemization of one well-known synthetic pyrethroid, deltamethrin, is described in J. Ag. Food Chem., Volume 25, page 1385 (1977). Racemization of pyrethroids, particularly deltamethrin, occurs frequently when they are formulated with carbamate compounds, possibly being caused or accelerated by impurities in the carbamate. The overall pesticidal activity of the mixture is thereby unacceptably reduced, negating the benefits that could otherwise be obtained by combining the two pesticides. Accordingly, an improved mixture in which the broad activity of the mixture is retained would be desirable.